1. Field of the Invention
This invention relates to a method and apparatus for inducing crystallization in a blown thermoplastic article. More particularly, the invention relates to thermally conditioning a blown article of polyethylene terephthalate so that only those portions of the bottle which have been molecularly oriented will be heated, in order to produce a clear article having enhanced physical properties.
2. The Prior Art
Various prior art patents have disclosed methods of producing biaxially oriented polyethylene terephthalate film which is subjected to a heat setting treatment for crystallization. Examples of such patents are U.S. Pat. Nos. 3,177,277, 2,995,779, 3,107,139, and 2,823,421 and British Pat. Nos. 851,874, 915,805, and 1,803,665, all incorporated by reference. Even more recently, U.S. Pat. No. 3,733,309, which is also incorporated by reference, has disclosed a blown polyethylene terephthalate bottle that may be heat treated.
However, the prior art has not taken into consideration that a blown polyethylene terephthalate bottle will have different degrees of molecular orientation along its axial dimension. Non-discriminate heat treating of the complete bottle would induce spherulitic crystal growth in non-molecularly oriented portions of the bottle. The resulting bottle, having opaque, brittle portions, would be commercially and structurally undesirable. Additionally, the prior art has not suggested how to heat treat a blown bottle while preventing distortion of the bottle due to the shrinkage of the material during heat treatment.
Accordingly, the prior art has not recognized the potential problems and adverse effects in heat treating blown thermoplastic bottles and has not suggested any solutions for those problems.